The most common hot-cathode ionization gauge is the Bayard-Alpert (B-A) gauge. The B-A gauge includes at least one heated cathode (or filament) that emits electrons toward an anode, such as a cylindrical wire grid, defining an anode volume (or ionization volume). At least one ion collector electrode is disposed within the ionization volume. The anode accelerates the electrons away from the cathode towards and through the anode. Eventually, the electrons are collected by the anode.
In their travel, the energetic electrons impact gas molecules and atoms and create positive ions. The ions are then urged to the ion collector electrode by an electric field created in the anode volume by the anode, which may be maintained at a positive 180 volts, and an ion collector, which may be maintained at ground potential. A collector current is then generated in the ion collector as ionized atoms collect on the ion collector. The pressure of the gas within the ionization volume can be calculated from ion current (Iion) generated in the ion collector electrode and electron current (Ielectron) generated in the anode by the formula P=(1/S) (Iion/Ielectron), where S is a constant with the units of 1/Torr (or any other units of pressure, such as 1/Pascal) and is characteristic of gas type and a particular gauge's geometry and electrical parameters.
The operational lifetime of a typical B-A ionization gauge is approximately ten years when the gauge is operated in benign environments. However, these same gauges fail in hours or even minutes when operated at high pressures or in gas types that degrade the emission characteristics of the gauge's cathodes.
In general, two processes may operate to degrade or destroy the emission characteristics of the gauge's cathodes. These processes may be referred to as coating and poisoning. In the coating process, other materials which do not readily emit electrons coat or cover the emitting surfaces of the gauge's cathodes. The other materials may include gaseous products of a process occurring in a vacuum chamber. The other materials may also include material removed or sputtered off from surfaces of the gauge that are at or near ground potential when ionized atoms and molecules impact these surfaces.
For example, heavy ionized atoms and molecules, such as argon, from an ion implant process, may sputter off tungsten from a tungsten collector and stainless steel from the stainless steel shield located at the bottom of the ionization gauge. As the pressure increases, there is an increase in density per unit volume of the argon atoms and, as a result, more material from the ionization gauge surfaces is sputtered off. This sputtered material, such as tungsten and stainless steel, may then deposit on other surfaces of the ionization gauge that are in a line-of-sight, including the cathodes. In this manner, the electron emission characteristics of the cathodes are degraded and even destroyed.
In the poisoning process, the emitting material of the gauge's cathodes may chemically react with gasses from a process occurring in a vacuum chamber so that the emitting material no longer readily emits electrons. The emitting material of the cathodes may include (1) an oxide-coated refractory metal that operates at about 1800 degrees Celsius or (2) nominally pure tungsten that operates at about 2200 degrees Celsius. The oxide coating may include yttrium oxide (Y2O3) or thorium oxide (ThO2) and the refractory metal may include iridium.
In one example, process gasses can chemically react with a cathode's oxide coating to degrade or destroy the cathode's ability to emit electrons. Specifically, when an yttrium oxide-coated cathode or a thorium oxide-coated cathode is heated, the yttrium or thorium atoms diffuse to the surface of the cathode and emit electrons. Process gasses can continually oxidize the yttrium or thorium atoms and dramatically reduce the number of electrons generated by the cathode.
Users do not want to stop their process to change gauges (or cathodes for gauges with removable cathodes) if they don't have to because that means down time, rework time, re-commission time, re-validate time, and so forth. Users would prefer to change gauges at their convenience, for example, when they do their preventative maintenance work. It is at this point that the user changes the ionization gauge and starts over with a new ionization gauge having new cathodes.
In order to increase the overall operational lifetime of an ionization gauge, second, backup or spare cathodes have been added to ionization gauges. The spare cathode may be a second half of a cathode assembly that includes two halves electrically tapped at a mid-point. In multi-cathode hot-cathode ionization gauges, gauge electronics or a gauge controller may operate one cathode at a time. For example, the gauge controller may use a control algorithm that allows the ionization gauge to alternate automatically or manually between the emitting and spare cathodes. However, in some applications, the electron emitting surface of the cathodes not being used can become poisoned and/or coated by a process. As a result, the ionization gauge control circuitry may turn off if it cannot cause the cathode to generate a desired electron emission current. Also, the cathode may become an open circuit (i.e., “burn out”) if the control circuitry overpowers the cathode in order to begin and sustain a desired electron emission current from the cathode surface.